1. Field of the Invention
The present invention relates to a tool unit comprising a tool holder and a tool body mounted thereto. The tool unit is disengageably retained by a spindle of a machine tool or the like. More particularly, the invention relates to a tool unit featuring high rigidity and enabling the tool body to be fixed to the tool holder with high precision.
2. Description of the Background Art
Machine tools, such as machining centers and the like, require automatic change of tools because a workpiece is typically subjected to different machining processes. Accordingly, such machine tools are arranged such that a variety of tool units are previously accommodated in a tool magazine and fed to an automatic tool changer. The tool changer, in turn, replaces a tool unit mounted to a spindle of a machine tool with another tool unit from the tool magazine. Each such tool unit comprises a tool holder with a common shank portion to be disengageably secured to the spindle, and a tool body mounted to the tool holder.
Various modes of mounting the tool body to the tool holder of the tool unit, in addition to a typical collet chuck system, are known in the art. In a mode shown in FIG. 8, for example, a tool holder 71 and a tool body 74 are fixed to each other by means of fixing means, such as a bolt 77. The tool holder 71 has an end surface 72 abutted against an end surface 75 of the tool body 74. A protrusion 73 formed on the end surface 72 of the tool holder 71 is fitted in a recess 76 formed in the end surface 75 of the tool body 71. In a mode shown in FIG. 9, a tapered bore 85 formed in a tool body 84 is tightly fitted on a tapered portion 82 of a tool holder 81. The tool body 84 is firmly secured to the tool holder 81 by means of a nut 86 firmly screwed on an external thread 83 extending from an end surface of the tapered portion 82 of the tool holder 81.
Recently, grinding tools and cutting tools have made dramatic progress. For example, CBN tools and diamond tools, such as end mills, grinding wheels and the like, employ edges or abrasive grains formed of CBN (cubic boron nitride) or diamond which are capable of cutting or grinding workpieces through superfast rotation. In this connection, the machine tools are adapted to rotate their spindles at extremely high speeds. Therefore, unless the CBN tool bodies are mounted with high precision to the tool holders, the CBN tools will suffer serious damage by running out of the tool bodies during superfast rotation. This hinders the inherent performance CBN tools.
Unfortunately, it is difficult for the aforementioned collet chuck system to assure high precision mounting of the tool body to the tool holder. Furthermore, because of its mechanism, the system suffers a risk of loosening between the tool holder and the toll body during fast rotation.
In the mode of FIG. 8, a gap is formed at the fitting portion of the tool holder and the tool body, resulting in the tool body running out from the tool holder to a higher degree. It is also difficult in this embodiment to form the end surfaces perpendicular to the axis of the tool.
As to the mode of FIG. 9, it is difficult to fabricate the tool holder and tool body in such a manner as to minimize the running out of the tapered portion and blade edge and to maintain constant axial dimensions.
The running out of the tools would be eliminated by integrally forming the tool body and the tool holder. However, providing a tool unit integrating the tool body and tool holder, and fabricating tool unit for each tool of a different configuration, results in increased overall production costs. In case of failure, the whole tool unit must be replaced, which results in increased replacement costs.
Alternatively, the tool body may be mounted on the tool holder by means of a shrinkage-fit method, as disclosed in U.S. Pat. No. 4,818,161. The shrinkage-fit method comprises the steps of, for example, (1) forming a shaft-like mount portion on the tool body and forming a fitting portion in the tool holder, wherein the fitting portion includes a fitting bore with a diameter slightly smaller than that of the mount portion; (2) heating the fitting portion to cause expansion of the fitting bore and then pressing the mount portion into the fitting portion; and (3) cooling the fitting portion to cause contraction of the fitting bore thereby establishing a shrinkage-fit between the mount portion and the fitting portion. The tool body and the tool holder are thus firmly secured to each other, and can assure a run-out precision close to that of a tool unit in which the tool body and tool holder are integrated.
Unfortunately, shrinkage-fitting the tool body on the tool holder results in a cumbersome process for mounting/dismounting the tool body to/from the tool holder. In addition, the tool body may be shrinkage-fitted on the tool holder as inclined or axially displaced relatively to the tool holder.